The present invention relates to a resin compound and to an adhesive film with excellent heat resistance, electrical properties, adhesion to metal and, at the same time, low elastic modulus; and the invention also relates to a circuit board and an assembly structure using these elements. The resin compound and adhesive film of the invention are applicable widely to the surface circuit layer and/or stress compliance layer of a circuit board. Because the circuit board of the invention has a low modulus of elasticity, it is most suitable for flip chip technology.
The latest technical innovations are remarkable in providing electronic devices, such as personal mobile data terminals, that are smaller in size, lighter in weight and higher in performance. As an assembly of these electronic devices, flip chip technology or a chip scale package (CSP) structure that makes it possible to realize high-density and compact packaging has come to be employed instead of conventional resin encapsulated semiconductor packaging using a resin compound made of epoxy resin and an inorganic filler.
As indicated above, a high-density compact package is frequently made of a grid array structure because, when the package is mounted on a circuit board, a number of pins must be arranged in a small area as external terminals for the package. Since there is a technical limitation to high-density packaging on a conventional circuit board for the above reason, a board with a surface laminate wiring circuit that has no through-hole, but comprises via-holes only, has been in use. As a material for forming a fine wiring circuit on a board surface, there are available a photo-via material, on which a wiring circuit is formed by exposure and development processes, and a laser-via material, on which holes are machined by laser beam or the like. Brief technology of the above type has been disclosed, for example, in xe2x80x9cPrinted Circuit Board Applicable to High-Density Packagingxe2x80x94Present Status and Problems of Build-up Circuit Board Technologyxe2x80x9d of xe2x80x9cElectronic Materialxe2x80x9d issued by Industrial Research Association, October 1998 issue.
Generally speaking, a material with high heat resistance, typically such as a polyimide, has a high modulus of elasticity. For this reason, if a packaging device with low coefficient of thermal expansion and high modulus of elasticity, such as a flip chip, is mounted on a polyimide circuit board, a question as to its reliability arises because of possible disconnection due to the thermal stress caused between the board and the device.
On the other hand, because the heat resistance of a rubber-added material with a low modulus of elasticity is generally lower, a question arises concerning the heat resistance against high-temperature bath solder (lead free solder) at about 300xc2x0 C.
The present invention provides a resin compound, which causes very little thermal stress even when a flip chip is mounted, which has a low modulus of elasticity, and which has a high heat resistance sufficient for solder floating at 300xc2x0 C., and the present invention also provides an adhesive film, a circuit board, and an assembly structure using the resin compound.
The means of solving the above problems in accordance with the present invention are as follows.
The first aspect of the invention involves a resin compound containing (A) a polyamide imide with siloxane bond, (B) an acrylic polymer with weight-average molecular weight of 500,000 or more, (C) a thermoset resin, and (D) a solvent.
The second aspect of the invention involves an adhesive film having the above-mentioned resin compound. This adhesive film provides an adhesive material that has both a low modulus of elasticity and high heat resistance, and also provides excellent adhesion to metal and electrical properties.
The use of (A) a polyamide imide with siloxane bond as an essential component in accordance with the invention makes it possible to maintain a low modulus of elasticity without lowering the heat resistance. The polyamide imide with a siloxane bond has superior solubility to solvent, as compared to the polyimides having a similar heat resistance, and it also has excellent miscibility with other polymers. It is a suitable component for realizing the effect of the resin compound of the present invention.
In addition, since the component (A) above has a siloxane bond, it has a higher miscibility with other components of the invention, including acrylic polymer and epoxy resin. As a result, a uniform film with no layer separation can be provided in a broad range of composition (mixture).
Particularly, in combining with a high molecular-weight acrylic polymer, the miscibility is an important issue. The siloxane bond content in the acrylic polymer structure is preferred to be 20 to 80 weight %. If it is less than 20 weight %, the miscibility decreases and a uniform resin compound with acrylic polymer cannot be obtained. If it exceeds 80 weight %, the adhesion to metal decreases, and, when a fine wiring circuit is formed, a lower reliability.
In accordance with the present invention, by using a polyamide imide containing a siloxane bond, it is possible to realize excellent miscibility with a high molecular-weight acrylic polymer, and, at the same time, to drastically lower the modulus of elasticity relative to that of conventional polyamide imide.
In accordance with the invention, (A) a polyamide imide with a siloxane bond represents a condition where, for example, an amide bond or an imide bond is bonded with the siloxane bond.
To be specific, a polymer that at least contains an amide bond, an amide bond as the main chain of an imide precursor, and the following bond unit is preferred:
xe2x80x94NHxe2x80x94COxe2x80x94Xxe2x80x94Zxe2x80x94 or xe2x80x94NHxe2x80x94COxe2x80x94Xxe2x80x94COxe2x80x94NHxe2x80x94Yxe2x80x94Zxe2x80x94
(X and Y in the above formula are an organic group including an aliphatic or aromatic group, and can be equal or different. Z is a siloxane bond.)
Besides, a polymer that contains a siloxane bond as the end group of an amide bond and an amide bond as the main chain of an imide precursor as follows, for example, is also effective:
xe2x80x94NHxe2x80x94COxe2x80x94Xxe2x80x94Z or xe2x80x94NHxe2x80x94COxe2x80x94Xxe2x80x94COxe2x80x94NHxe2x80x94Yxe2x80x94Z
(X and Y in the above formula are an organic group including an aliphatic or aromatic group, and can be equal or different. Z is a siloxane bond.)
In accordance with the present invention, by using (B) a acrylic polymer with a weight-average molecular weight of 500,000 or more, it is possible to realize a low modulus of elasticity without lowering the heat resistance. If an acrylic polymer with a weight-average molecular weight of less than 500,000 is used, sufficient properties, including heat resistance and electrical properties, cannot be attained. The use of a high molecular-weight polymer with weight-average molecular weight of 500,000 or more makes it possible to attain stable properties.
The mix ratio of the above component (A) to component (B) is preferably 3:1 to 1:5 by weight. In mixing the two, from the point of view of lowering the modulus of elasticity, increasing the mix ratio of the component (B), i.e. the acrylic polymer, is preferable so as to increase the resolution in film forming and the adhesion to metal. However, increasing the component (B), i.e. the acrylic polymer, in excess of the mix ratio 1:5 results in insufficient properties, including heat resistance and electrical properties.
It is also preferable when the glass transition temperature of the acrylic polymer is 0xc2x0 C. or less. Because of this, the modulus of elasticity under a range of 0 to 150xc2x0 C., a normal operating environment, can be maintained low.
Since use of a combination of the above components (A) and (B) in accordance with the present invention is not sufficient enough to maintain the properties at high temperature, it is preferred to utilize a component that forms a cross-linked structure so as to maintain the solder heat resistance at 300xc2x0 C. or above.
A thermoset resin is a component that forms a cross-linked structure. It is available in the form of, for example, an epoxy resin, a maleimide resin, a cyanate resin, an isocyanate resin, a phenol resin, an allyl resin, an unsaturated polyester resin, a butadiene resin, a silicone resin, a vinyl ester resin, and a silicone resin. Among all these, a epoxy resin that comprises an epoxy compound, a hardener containing a phenol group, and an imidazole curing accelerator is the most preferable.
Because a epoxy resin before cross-linking is generally a low molecular-weight compound or low molecular weight polymer, it contributes to improved wettability in bonding a film with various materials and brings about an effect of increasing the adhesion to metal.
In accordance with the present invention, the addition of the thermoset resin component (C) is preferred to be 5 to 50 weight % of the entire film. If it is less than 5 weight %, the effect of utilizing a cross-linked structure formed by the thermoset resin component cannot be produced sufficiently, and, consequently, sufficient heat resistance at high temperature cannot be attained. If it exceeds 50 weight %, the modulus of elasticity tends to increase, and, consequently, the resolution in film forming is negatively affected.
The epoxy resin of the invention can be a compound used in a semiconductor sealing material or printed circuit board material. It is available in the form of, for example, a phenol novolac type epoxy resin, a cresol novolac type epoxy resin, a bisphenol type epoxy resin, including bisphenol A, bisphenol F and bisphenol S type epoxy resins, a multi-functional epoxy resin of phenol resin or cresol resin, and a bi-functional or tri-functional epoxy resin having a naphthalene skeleton, antracene skeleton or dicyclo-pentadiene skeleton.
A publicly known compound is applicable as the hardener for the above-mentioned epoxy resins. Among all these, a compound having two or more phenolic hydroxyl groups is preferable. It is available in the form of, for example, a phenol novolac resin, a condensation polymer of phenol and aralkyl ether, a bisphenol resin, an o-cresol novolac resin, and a poly p-vinyl phenol.
As needed, a publicly known compound is applicable as the curing accelerator. It is available in the form of, for example, an imidazole compound, a tertiary phosphate compound, a quaternary phosphate compound, a complex of a tertiary phosphate compound and a borate compound, a complex of a quaternary phosphate compound and a borate compound, a tertiary amine, a borate complex salt, Lewis acid, an inorganic acid, a short-chain amide, a dihydrazide, a titanic ester, and a cationic or anionic catalyst. Among all these, an imidazole compound is preferable because it excellently adds to the heat resistance, adhesion, and shelf life of the material.
If the physical properties of the adhesive film of the present invention after heating and curing allow the modulus of elasticity at 50xc2x0 C. to be less than 1 GPa, the thermal stress caused by temperature variation can be reduced drastically at the time when a flip chip is mounted on the film, and the reliability under a thermal cycle and other conditions can be improved tremendously.
In accordance with the present invention, the solvent component (D) has no particular limit provided that the thermoset polymer as the above components (A) and (B) and the epoxy resin, hardener, and curing accelerator as the component (C) can be evenly mixed in it. A preferable solvent is, for example, acetone, methyl ethyl ketone, methyl isobutyl ketone, methyl cellosolve, y-butyrolactone, N,N-dimethyl formamide, N-methyl-2-pyrrolidone, toluene, dimethyl acetamide, cyclohexanone, or isopropyl alcohol, and a mixed solvent of two or more of the above is allowable.
An additive, such as an inorganic filler, colorant. or flame retardant can be added so long as the properties of the resin compound and the adhesive film of the invention are not affected.
The third aspect of the invention involves a metal-clad adhesive film that utilizes the resin compound of the first aspect of the invention as an insulation layer. By use of the metal-clad adhesive film as wiring material for the below-mentioned build-up structure, the process can be simplified.
The resin compound of the present invention is bonded onto a metal foil by a process in which a specified amount of the resin compound containing solvent is applied on a metal foil and then the metal foil and the resin film are bonded together as the solvent is removed by heating, depressurizing, or other processing. The metal foil has no specific limit provided that it serves as a conductor forming a circuit, but copper is preferable. A metal-clad adhesive film is available in a structure where a metal foil is formed on one side of the film or on both sides of the film.
The fourth aspect of the present invention involves a circuit board having a built-up structure where the adhesive film of the second aspect of the invention is arranged on the circuit board and the circuit layer of the circuit board is electrically connected to external terminals. The adhesive film to be arranged on the circuit board can be either a simple adhesive film or the above-mentioned metal-clad adhesive film.
If a simple adhesive film is arranged on a circuit board, the circuit board having a built-up structure can be obtained by a process in which, after holes for via-hole connection are drilled at specified positions using a laser machine or the like, the board is subjected to roughing and conductor plating so as to form a wiring circuit and attain the continuity to the lower circuit board by means of connection through the via-holes. By repeating this process as needed, a built-up circuit board having a multi-layer structure can be provided.
If a metal-clad adhesive film is arranged on a circuit board, holes are made at necessary positions of the metal foil beforehand by etching or other means and then holes for via-hole connection are drilled in the film by a laser machine or the like, utilizing the metal foil as a mask. Then, the holes for via-hole connection are made conductive by plating, and lastly a wiring circuit is etched on the metal foil. Thus, a circuit board having a built-up structure is formed. By repeating this process as needed, a built-up circuit board having a multi-layer structure can be provided.
The fifth aspect of the present invention involves an assembly structure in which a flip chip is mounted on the circuit board having a built-up structure according the fourth aspect of the invention. The surface of a circuit board with the built-up structure to mount a flip chip is made of an insulation layer with a low modulus of elasticity. The use of the assembly structure can drastically reduce the thermal stress to be caused by the difference in the coefficient of thermal expansion between the flip chip and the circuit board. Thus, it becomes possible to secure connection reliability under a thermal cycle and other conditions without using an underfill resin that was needed in the prior art.
The present invention can reduce cost, including the material and process cost, tremendously. Because no underfill resin is needed, the invention makes it possible to perform repairs more easily than before and can contribute greatly to improvement of the yield. dr
FIG. 1 is a diagram showing a cross-sectional view of an adhesive film.
FIG. 2 is a diagram showing a cross-sectional view of an adhesive film on one side of which a metal foil is arranged.
FIG. 3 is a diagram showing a cross-sectional view of a circuit board of a build-up structure on which an adhesive film is arranged.
FIG. 4 is a diagram showing a cross-sectional view of a circuit board having a built-up structure on which layers of adhesive films are arranged in a multi-layer structure.
FIG. 5 is a diagram showing a cross-sectional view of a circuit board having a built-up structure on both sides of which an adhesive film is arranged.
FIG. 6 is a diagram showing a cross-sectional view of an assembly structure in which a flip chip is mounted on a circuit board having a built-up structure.
FIG. 7 is a diagram showing a cross-sectional view of an adhesive film on both sides of which a metal foil is arranged.